Clutches are well known mechanical components for selectively coupling a driving member with a driven member, wherein the "members" are typically coaxial rotating components. There are many different types of clutches, but virtually every clutch can be categorized either as a "positive" clutch or a "friction" clutch. When the driving and driven members of a clutch are interlocked by the mechanical engagement of teeth or lugs, the clutch is said to be positive. A common type of positive clutch includes a pawl which engages a ratchet. By contrast, friction clutches transmit torque frictionally, using simple rotating friction plates, pads, or the like.
Clutches can also be categorized depending on whether the driven member can overrun the driving member when the former rotates faster than the latter. Overrunning clutches, those which allow the driven member to rotate at a higher speed than the driving member, are useful in many applications, e.g., coupling starter motors to internal combustion engines.
Still another feature of some clutches is the ability to "freewheel." Although the terms overrunning and freewheeling are sometimes used interchangeably in describing clutches, in the present application the term freewheeling is specially defined. As used herein, a "freewheeling" driven member can rotate relative to a driving member when the latter is not being rotated in the normal drive direction(s).
To illustrate, the terminology discussed above can be applied to the traditional ratchet clutch. In one type of ratchet clutch, the driving and driven members each include teeth and the opposing toothed members are urged together by some type of spring. The driving member can be rotated in either direction but significant torque is transmitted to the driven member in one direction only. In the opposite direction the teeth slip over one another, thereby generating the well-known clicking sound associated with ratchet-style clutches. Thus, the traditional ratchet clutch can be categorized as a positive clutch which permits overrunning and which permits freewheeling in one direction only.
Having defined the relevant terms, the present invention and the prior art can now be succinctly discussed. One embodiment of the present invention is a positive clutch which provides bidirectional freewheeling, and which preferably provides for overrunning. The clutch is actuated or engaged using friction, but torque is not transmitted from the driving member to the driven member in traditional friction clutch manner, i.e., through the use of friction pads or the like.
Various designs for friction-actuated, positive, bidirectional freewheeling clutches have been proposed. For example, U.S. Pat. No. 3,463,280, issued to G. J. Hoffman et al., discloses a clutch assembly including a driving shaft, a driven hub, a housing, and a rocking pawl mounted on a rather delicate projection extending from a collar mounted on the driving shaft. The Hoffman pawl is "external" in the sense that it is not contained within a notch or slot in the driving member, but instead is carried externally thereto on a collar. When the driving member is rotated relative to the housing the "external" pawl cocks and engages teeth or notches in the driven hub. The pawl is cocked by a disk which frictionally engages the housing. The frictional drag on the pawl causes it to cock and engage the driven hub to thereby allow torque to be transmitted from the driving shaft, through the pawl, and to the driven hub. On the other hand, if the driving member is stationary, the driven member can freewheel in either direction since the pawl is not cocked in the absence of rotational movement of the driving shaft relative to the housing.
While the Hoffman clutch described above may be conceptually satisfactory, it possesses several shortcomings. For example, the delicate pawl carrier which extends from the collar mounted on the driving shaft may tend to fail when subjected to substantial loading on a regular basis. The rather slender pawl carrier must absorb and transmit all of the torque from the driving member to the driven member. Also, the Hoffman "external" pawl design is space-consuming in that it requires an additional collar connected to the shaft and a pawl carrier, and must accommodate external rocking or pivoting of the pawl relative to the driving member.
A design conceptually similar to the Hoffman clutch is disclosed in U.S. Pat. No. 967,006, issued to R. C. Feile. The Feile clutch assembly includes a driving axle, a driven wheel assembly, a pawl pivotally pinned to a friction disk, and a journal bearing in contact with a stationary housing which ultimately supports the entire assembly. The pawl includes a rather fragile lower extension which fits within a notched member extending from the driving shaft. The Feile clutch functions quite like the Hoffman clutch: When the driving shaft rotates the pawl is cocked by virtue of the drag on the friction disk. The cocked pawl engages an internal tooth on the wheel assembly.
The Feile clutch, like the Hoffman clutch, is conceptually suitable for some applications. However, it too includes a rather bulky "external" pawl, i.e., a pawl mounted externally to the driving shaft rather than internally within a notched or relieved area. Also, the lower projection on the pawl and the pawl pin are likely to wear, if not fail, over a long period of time. Finally, the Feile clutch includes an excessive number of parts.
One embodiment of the present invention is a friction-actuated, positive, bidirectional-freewheeling clutch which addresses the problems associated with functionally similar prior art clutches.
Another embodiment of the invention is a power implement employing such a clutch. One particularly advantageous application is in the traction drive system of a self-propelled lawn mower. A self-propelled walk-behind lawn mower typically includes a rotating blade mounted beneath a housing which is supported by a plurality of wheels. Mounted atop the housing is a prime mover (e.g., internal combustion engine) which can be selectively coupled to the blade and to one or more traction drive wheels. A traction drive clutch interconnects the prime mover and the drive wheel(s).
In a lawn mower drive system employed by Honda and at least partially disclosed in U.S. Pat. No. 4,554,780, issued to Umeno et al., a first drive shaft extends between the prime mover and a gear box. A "dog" clutch couples the output or "jack" shaft of the gear box to a second drive shaft which ultimately drives the traction wheels. A control wire attached to the dog clutch connects to a control lever mounted on the lawn mower's handle. The second drive shaft is connected to a pair of rear drive wheels through a pair of one-way ratchet clutches. When the dog clutch is engaged there is a direct and sudden mechanical connection between the prime mover and the rear drive wheels. This results in "jack rabbit" starts which can actually cause the lawn mower to rear back on its traction wheels like a high-powered dragster.
An improved lawn mower drive system has been employed by The Toro Company, assignee herein. In the Toro system, a smooth V-belt couples the prime mover to a gear box pivotally mounted on the mower's housing. The output shaft of the Toro gear box is connected through a pair of one-way clutches to a pair of rear drive wheels. The one-way clutches provide a simple differential to allow the drive wheels to rotate at different speeds during a turn. A clutching action between the prime mover and the traction wheels is achieved by virtue of the pivotal connection between the gear box and the housing: when it is desirable to engage the drive wheels the gear box is pivoted away from the prime mover through the use of a control wire/control lever assembly. The prime mover/traction wheels interconnection can be "feathered" by gradually increasing the distance between the prime mover pulley and the gear box pulley. Thus, with the Toro system, uncomfortable and potentially dangerous jack rabbit starts are eliminated.
While the Toro system has proven to be superior to the Honda system in terms of eliminating jack rabbit starts, the Honda system provides for easier freewheeling of the drive wheels. Freewheeling of the traction wheels is important because it allows the mower to be freely and easily moved in both directions during trimming operations (when the blade is rotated but the traction wheels are not) and during transportation of the mower. As discussed above, the Honda gear box jack shaft is connected to the gears through the use of a dog clutch. When the dog clutch is disengaged, the only drag on the drive wheels is that imposed by the ball bearings which carry the jack shaft. The drive wheels can thus smoothly and easily freewheel in either direction. In fact, the "pull back" force on some Honda mowers is only about 5 pounds when the dog clutch is disengaged.
By contrast, when the Toro V-belt type clutch is disengaged and the gear box is in neutral the pull back force on new Toro mowers is typically about 19 to 21 pounds. This larger pull back is primarily attributable to the fact that Toro employs 3-to-1 speed reduction gearing at the drive wheels. Also, to reduce costs and increase reliability, Toro uses bushings rather than ball bearings in the primary gear box. Of course, when the gear box is not in neutral the pull back force on the Toro mowers is even higher. Trimming operations are thus hindered and the drive wheels occasionally skid, rather than roll, over the grass.
The clutch of the present invention could be situated between the gear box output shaft and the traction wheels of a mower like the Toro mower described above, so that when the output shaft is driven by the prime mover the wheels are also driven, and so that when the output shaft is no longer driven by the prime mover the traction wheels can readily freewheel in either direction relative to the drive shaft.
Therefore, the present invention, in addition to being directed toward a simple, compact bidirectional-freewheeling clutch, is also directed toward a powered implement employing such a clutch. In a preferred embodiment of such an implement, a pair of such clutches is situated between a driving shaft of a lawn mower and a pair of drive wheels so that freewheeling of the drive wheels is permitted when the driving shaft is not being rotated by the prime mover.